1. Field of the Invention
The present invention relates in general to a redundancy suppression of an image signal and in particular concerns a signal compression method which allows the redundancy suppression of a multilevel (gray scale) digital image signal to be effected with an enhanced compression efficiency.
2. Description of the Prior Art
Heretofore, the redundancy suppression of a binary signal has been adopted in the field of facsimile or FAX and the like technologies with a view to reducing a time required for transmission of information.
On the other hand, in an optical character reader (hereinafter referred to simply as OCR) or the like which is capable of reading document information at a high speed and thus requires an image signal buffer memory for storing once therein all the image information of a whole document for subsequent processings such as segmentation and recognition processings, the redundancy suppression of the image information signal is desired from the standpoint of decreasing the capacity of the buffer memory. In connection with the OCR systems, it is noted that scan is in general effected twice by changing the digitizing threshold level (i.e. so-called re-scan). Thus, in the case of the OCR in which all the image information of a whole document is once stored in the buffer memory as described above, whole signals having a plurality of levels corresponding to the number of the digitizing threshold levels as employed have to be stored simultaneously in the buffer memory. For this reason, the redundancy suppression of the multilevel signal is required.
In the OCR, the scan is usually effected up to twice, which means that the digital signals having three levels have to be handled. In the case of a four-level image signal, each of picture elements produces one of the four levels and thus provides two-bit information. In the case of this type OCR, there arises thus a demand for the technique which allows the four-level signal to be compressed.
A signal compression which is frequently adopted for the compression of the binary signal and referred to as B-code technique can be so expanded as to be applied to the multilevel signal, which however involves such a problem that the compression rate or efficiency is very low, as will be described hereinafter. As to the B-code, reference may be made to Suehiro and Matsumoto's "Facsimile Signal Band Compressing System" published by Nippon Hyoron-sha, Dec. 1977, pages 1101 and 1970 and Meyre's et al. "Optimum Run Length Codes" IEEE Trans.-COM-22, 6, p. 826 (1974).